This invention relates to waveguide transmission systems and, more particularly, to a system for locating faults in an overmoded waveguide transmission line.
A waveguide fault which is induced by mechanical damage from an outside source such as an accidental dig-up that results in a crushing or rupture of the waveguide protective sheath, is readily locatable by a gas pressure fault location system and visual inspection of route for the dig-up. Internal waveguide faults, however, are not locatable with a gas pressure fault location system or route inspection. For example, one type of internal fault that causes transmission impairment is a weld failure at a waveguide joint. Similarly, a delamination of the dielectric liner can cause some level of transmission degradation depending upon the extent of delamination. Also, a stress or chemical corrosion of the copper plating due to processing faults will result in a transmission loss due to the ohmic loss increase of the guide's inner surface. An electrical fault location system is required to precisely locate these interior waveguide faults that increase the transmission loss of the waveguide.
Standard waveguide fault location techniques which locate the fault by measuring the reflections generated therefrom can not be employed in an overmoded waveguide transmission line. Since a waveguide fault in an overmoded waveguide excites other transmission modes in which the forward scattering of energy is dominant, the amount of energy transmitted in the backwards direction from a waveguide fault is not sufficient to determine its location.
A fault location system for an overmoded waveguide transmission line is described in U.S. Pat. No. 3,750,012 issued July 31, 1973 to R. G. Fellers and L. W. Hinderks. In that system, reflectors are periodically spaced within a waveguide to enable the detection of an increased transmission loss between two consecutive reflectors. In response to such an increased loss, a fault can be determined to exist between two reflectors. In a waveguide system of significant length, however, a large number of reflectors must be inserted into the waveguide. Since, however, each reflector cumulatively contributes to the forward loss of a transmitted data or voice signal, either the number of reflectors inserted in the waveguide must be limited or the reflectors must be designed to be responsive only within a specific frequency band. Therefore, either a fault can be localized within only a long section of waveguide, or the reflectors must be precisely engineered, the latter prohibitively increasing the cost of a waveguide fault location system. In addition, in the latter situation a frequency band must be designated for only testing purposes, thereby reducing the available bandwidth for data and voice transmission. In either situation, however, a fault can be determined to exist only between two fixed reflectors.